1. Field of the Invention
The invention relates to an automatic transmission.
2. Description of the Related Art
In the automatic transmission of the prior art, the rotation as generated by an engine is transmitted through a torque converter to a speed change unit and the speed-changed rotation is transmitted to drive wheels. The speed change unit is equipped with a gear unit composed of a plurality of gear elements for establishing a plurality of gear stages by applying/releasing frictional engagement elements including clutches and brakes for selective output of the rotation of the gear elements.
In order to lubricate and cool the rotary elements such as the gear elements and bearings, an oil pan provides an oil reservoir below the casing of the automatic transmission from which oil is taken up by an oil pump and fed to the individual rotary members. The oil, having lubricated and cooled the rotary members, is scattered by centrifugal force and then drops by gravity and is recovered in the oil pan.
In the automatic transmission of the prior art, however, the number of rotary members is extremely increased if a plurality of gear units are arranged in the automatic transmission casing to provide multiple stages, so that the amount of oil required to be fed to the individual rotary members is accordingly increased.
When the oil becomes more viscous and less fluid at a low temperature, the level of oil is lowered in the oil pan because the oil is left adhering to the surfaces of the rotary members, in spite of the centrifugal force, so that the amount of oil recovered in oil pan is accordingly reduced. While it is conceivable to reserve a greater amount of oil in the oil pan, such an approach would not only raise the cost but would also cause loss of the lubricity of the oil itself to lower the efficiency of the automatic transmission. If the oil is degraded or aerated, moreover, its lubricating and cooling capabilities decline.